1. Field of the Invention
The present invention relates to an ND filter, an iris device and an optical apparatus having the ND filter.
2. Description of the Related Art
Conventionally, in image pick-up apparatuses such as video cameras, an ND (neutral density) filter has been used in the light quantity reduction device thereof to prevent the aperture from becoming extremely small for the bright object field, thereby avoiding undesirable phenomena such as hunting and diffraction of light.
Specifically, there are known iris devices using an ND filter. For example, there are an iris reduction device using a step ND filter having multi-density ND film that vary stepwise as disclosed in Japanese Patent Application Laid-Open No. 2002-277612 and an iris device using a gradation ND filter whose density varies in a stepless manner over a large density variation range as disclosed in Japanese Patent No. 03621941.
In the case where a conventional ND filter is used in an iris device, a rather complex control is required.
Specifically, the difference between the photometry value on the light quantity transmitted through the iris device at a certain time and an appropriate transmitted light quantity is computed, and an aperture blade or an ND filter is operated in accordance with a predetermined algorithm based on the difference, whereby an appropriate exposure amount can be attained. In addition, to attain optimum exposure or to perform optimum exposure correction, the control algorithm may be changed depending on the object illuminance, or in some apparatuses, information on the set focal length of a zoom lens and information on the focusing position (length) of the lens are also used as control parameters.
As per the above, the control process involves very complex control factors.
Furthermore, when the transmitted light quantity is changed by moving the ND filter and varying the density of the ND filter, the diameter of the aperture is often small. Then a slight difference in the ND filter density results in a large difference in the transmitted light quantity.
Therefore, especially when the gradation ND filter whose density changes continuously at any position in accordance with the operation amount thereof is used to control the light quantity, it is very difficult to determine the relationship between the operation amount and the transmitted light quantity.
On the other hand, in the case of the step ND filter, it is sufficient to consider only the position at which the density changes. In other words, it is sufficient to consider only the ratio of the area of the lower density portion and the area of the higher density portion. Therefore, the control process is relatively easy, since it can be performed based on a concept similar to that in the case where a conventional uniform density ND filter is used.
However, in the case of the step ND filter, as light is considered as a wave, a difference in the optical thickness of ND film in the regions of the filter that the light passes causes a phase difference, which affects the image quality. Specifically, as disclosed in Japanese Patent Application Laid-Open No. 2004-253892 (see page 22 and FIG. 24), a transmission wave front phase difference affects the image quality.
This is because in the case of a kind of step ND filter in which the density is varied by changing the thickness of a ND film having the same constant refractive index, phases of light beams having passed through regions having different densities associated with different ND film thicknesses become different from each other to cause interference that weakens the light, thereby deteriorating the resolution. The deterioration of the image quality becomes maximum when the areas of the regions having different densities are equal to each other. If the difference in the ND film thickness between these regions or the transmission wave front phase difference is changed in that state, the MTF (modulation transfer function) value (representing axial optical characteristics) will change with a cycle λ.
The MTF value changes in the following manner. The MTF value decreases as the phase difference between the regions increases from zero, and the MTF value takes an extremal value at a phase difference of 2λ/4 and starts to increase, then takes an extremal value at a phase difference of 4λ/4 and starts to decrease again, then takes an extremal value at a phase difference of 6λ/4 and starts to increase, and takes an extremal value again at a phase difference of 8λ/4.
As compared to the gradation ND filter, the step ND filter suffers from deterioration in image quality caused by diffraction that occurs due to density discontinuity at the boundary between regions with different densities, especially in the case where the density difference is large.
As per the above, the step ND filter suffers from the problem of diffraction and transmission wave front phase difference, and the image quality is sometimes worse in cases where a step ND filter is used than in cases where a gradation ND filter is used.
However, control of an iris device is easier, and a control program can be developed in a shorter time in cases where a step ND filter is used than in cases where a gradation ND filter is used.